This invention relates to the production of diorganyloxyorganosilyl- or triorganyloxysilyl-terminated poly(diorganosiloxanes) by reaction of .alpha.,.chi.-dihydroxypoly(diorganosiloxanes) with alkoxysilanes in the presence of catalytically active strong alkali metal bases.
There are already several known processes for the production of the polymers according to the invention and formulations containing such polymers. The products are used for the production of one-component silicone pastes which cure in the presence of moisture to form elastomers which are referred to hereinafter as RTV-1C compounds.
U.S. Pat. No. 3,161,614 describes the reaction of .alpha.,.chi.-dihydroxypoly(diorganosiloxanes) with polyfunctional halosilanes, for example SiCl.sub.4 or CH.sub.3 SiCl.sub.3. The resulting halogen-containing polysiloxanes are then converted into di- or triorganyloxysilyl-terminated polysiloxanes by reaction with alcohols in the presence of acid-binding agents. The same patent specification also mentions the reaction of .alpha.,.chi.-dihydroxypoly(diorganosiloxanes) with alkoxysilanes in the presence of suitable catalysts, such as amines and metal carboxylates.
The production of RTV-1C compounds is described in EP 21 859 and in EP 69 256. According to these publications, .alpha.,.chi.-dihydroxypoly(diorganosiloxanes) are reacted with alkoxysilanes in the presence of amines to form the polymers according to the invention.
EP 70 786 describes the use of hydroxylamine derivatives instead of amines as catalysts.
Silanes of mixed functionality are also known and may be reacted with .alpha.,.chi.-dihydroxypoly(diorganosiloxanes) in contrast to pure organyloxy or organyloxyorganosilanes to form the polysiloxanes produced in accordance with the invention, even in the absence of catalysts. The silanes in question include alkoxyamidosilanes (DE-PS 1 247 646), alkoxyoximinosilanes (EP 98 369) and alkoxyacetoxysilanes (U.S. Pat. No. 3,296,195).
DE-PS 3 523 206 claims the use of ammonium carbamates, preferably (CH.sub.3)NH.sub.2 OCON(CH.sub.3).sub.2, as catalyst for the reaction of OH-terminated polysiloxanes with alkoxysilanes.
Mixtures of amines and carboxylic acids are described as catalysts for the same reaction in EP-PS 137 883.
All hitherto described processes for the production of the polysiloxanes mentioned have disadvantages. The process in which .alpha.,.chi.-dihydroxypoly(diorganosiloxanes) are reacted with halosilanes, followed by alcoholysis (U.S. Pat. No. 3,161,614), leads to polymers which contain corrosive ammonium salts and is complicated.
Alkoxysilanes of mixed functionality, which contain an amido, amino, oximino or carboxylato group in addition to the alkoxy groups, give the desired triorganyloxysilyl- or diorganyloxyorganosilyl-terminated poly(diorganosiloxanes) by reaction with .alpha.,.chi.-dihydroxypoly(diorganosiloxanes). However, the preparation of the silanes is generally expensive and the removal of the cleavage products necessitates separate process steps in the preparation of the formulations or is impracticable. However, the removal of the cleavage products formed from the silanes mentioned would be desirable for the formulation of chemical neutral, transparent polysiloxane compounds. Accordingly, it is preferred to react OH-terminated polysiloxanes with alkoxysilanes in the presence of suitable catalysts.
All the hitherto described catalysts or catalyst systems have the disadvantage that they necessitate long reaction times and elevated temperatures. In addition, the catalysts have to be used in substantial quantities and, in most cases, are difficult or even impossible to remove from the mixtures. This applies to amines, hydroxylamine derivatives and mixtures of amines and carboxylic acids. Although the process which uses carbamates, such as (CH.sub.3).sub.2 NH.sub.2 OCON(CH.sub.3).sub.2, is more suitable, it does involve the handling of considerable quantities of dimethylamine. In addition, it is confined to termination with methoxysilanes. Accordingly, it would be desirable to have a process which could be carried out simply in a short reaction time and which could even be applied to less reactive alkoxysilanes, for example ethoxysilanes.
The use of strong bases as catalysts would actually be logical. However, no such process has yet been described. On the contrary, strong bases, such as KOH or NaOH, were not considered suitable as catalysts in the prior art because they rapidly produced unwanted rearrangement reactions involving the polymer chain. For example, it is known that branched monoalkoxy-terminated polysiloxanes are formed from trialkoxy or tetraalkoxysilanes and cyclotetra(dimethylsiloxane) in the presence of strong bases, such as KOH or potassium siloxanolate (U.S. Pat. No. 2,909,549). .alpha.,.chi.-Dihydroxypoly(dimethylsiloxanes) also lead to the same product by reaction with methyl trimethoxysilane in the presence of KOH. However, it is not possible to produce a practicable RTV-1C system from monoalkoxy-terminated polymers such as these which contain the terminal group-OSi(CH.sub.3).sub.2 OCH.sub.3 for example.